1. Technical Field
The present invention generally relates to wireless communication systems, and particularly relates to signal timing measurements in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system.
2. Background
The 3rd-Generation Partnership Project is currently developing specifications for a next generation of wireless networks, as part of the so-called Long Term Evolution (LTE) initiative. Under the current plans, Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink. As will be familiar to those skilled in the art, OFDMA is a modulation scheme in which the data to be transmitted is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence in OFDMA based systems, the available bandwidth is sub-divided into several resource blocks or units as defined, for example, in 3GPP TR 25.814: “Physical Layer Aspects for Evolved UTRA”. According to this document, a resource block is defined in both time and frequency. According to the current assumptions, a resource block size is 180 KHz and 0.5 ms in frequency and time domains, respectively. The overall uplink and downlink transmission bandwidth can be as large as 20 MHz.
In order to simplify equalization in the Orthogonal Frequency Division Multiplexing (OFDM) receiver, as well as to avoid inter-carrier and inter-block interference, a cyclic prefix is used, wherein each transmitted OFDM symbol is prefixed by a copy of the last samples of the OFDM symbol. The cyclic prefix provides a time-domain buffer between a current OFDM signal and the previously transmitted OFDM symbol, thus avoiding inter-block interference. In addition, the cyclic prefix effectively transforms the linear convolution performed by the radio channel into a circular convolution. As a result of this latter effect, inter-carrier interference is eliminated, and equalization of the received OFDM signal is simplified. The length of the cyclic prefix is generally selected so that it will usually exceed the delay spread of the radio propagation channel (i.e., the time difference between the first and last arriving multipath signals in the channel impulse response).
In an LTE system, a mobile terminal (in 3GPP terminology, “user equipment”, or “UE”) performs various measurements to facilitate radio resource management (RRM) related tasks such as contention-free handover. A measurement that can be particularly useful for contention-free handover is a time difference between a first OFDM signal, from a serving base station, and another OFDM signal from a target base station. If the time difference from the mobile terminal's perspective is known, then the network can use this information to adjust the mobile terminal's transmission timing when it accesses the target cell at handover. This ensures that the mobile terminal's transmitted signal arrives at the target cell with the proper timing, e.g., at the correct slot and frame boundaries.
Time difference information may also be useful for other applications as well. For instance, time difference information for a serving cell and each of several neighbor cells (at least two, and preferably three or more) may be used by a serving cell to estimate the mobile terminal's position, using well-known triangulation techniques.
Similar measurements are performed in Wideband Code-Division Multiple Access (WCDMA) systems. In WCDMA, a mobile station measures the arrival times of pilot symbols originating from the serving cell and possible target cells. Subsequently, the differences between the arriving time of the pilot symbol of the serving cell and pilot symbols from possible target cells are calculated. More specifically, in WCDMA there are two such measurements performed on some known channel or pilot symbols, known as SFN-SFN type 1 measurements and SFN-SFN type 2 measurements. The former measures the time difference between start of the reception of the P-CCPCH (Primary Common Control Physical Channel) from the serving cell to the start of the reception of P-CCPCH from the target cell. For SFN-SFN type 2 measurements, the mobile terminal measures the time difference between the start of the reception of the CPICH from the serving to the start of the reception of the CPICH from the target cell.
In a CDMA-based system, measuring the arrival time of pilot symbols can be done in several ways. For instance, the receiver can correlate the received signal with a pre-determined sequence that is expected in the received CDMA signal, e.g., one or more pilot symbols or synchronization signals. The arrival time can be designated as the time where the first correlation peak occurs, corresponding to the shortest path in a multipath signal environment. Alternatively, the arrival time may be designated as the time where the largest correlation peak occurs, corresponding to the arrival of the strongest path.
Referring signal arrival times to the arrival time of one ray of a multipath signal fits very well to a CDMA transmission system where the mobile terminal tries to align itself with the first received or detected path in time. However, this approach is inappropriate for OFDM based systems because individual correlation peaks have little significance in an OFDM receiver.